This invention relates to a cathode-ray tube, and more particularly to a cathode-ray tube incorporating an electron gun assembly which compensates for dynamic astigmatism.
Generally, a color cathode ray art tube has an envelope as shown on FIG. 1. The envelope comprises a panel 1 and a funnel 2 joined to the panel 1. A phosphorous screen 3 (target) is provided on the inner surface of the panel 1. the screen 3 comprises striped or dot-like three-color phosphor layers for generating blue, green, and red light rays. A shadow mask 4 is provided in the funnel 2 and faces the phosphor screen 3. The shadow mask 4 has a large number of aperatures. The funnel 2 has a neck 5, in which an electron gun assembly 7 is provided. A deflection yoke 8 is mounted on the neck 5. The electron gun assembly 7 emits three electron beams 6B, 6G, and 6R. The yoke 8 generates a horizontal magnectic field and a vertical magnetic field. These magnetic fields deflect the electron beams 6B, 6G and 6R in horizontal direction and vertical direction, respectively. The electron beams 6B, 6G and 6R pass through the shadow mask 4, scanning the phosphor screen 3 in horizontal and vertical directions. A color image is thereby displayed on the panel 1.
A type of a color cathode-ray tube, known as a self-convergence, in-line-type color cathode-ray tube, is used widely. This cathode-ray tube comprises an in-line type gun assembly having three electron guns 7 which are arranged side by side in the same horizontal plane. The guns 7 emit a center electron beam 6B and side electron beams 6G and 6R. The side beam 6G is on one side of the center beam 6B, and the side beam 6R on the other side thereof. The three beams 6B, 6G and 6R travel in a horizontal plane. The electron gun assembly has a main lens section, in which a low-potential grid and a high-potential grid are arranged. Each grid has three beam-guiding holes. The center beam-guiding hole of the high-potential grid is concentric to that of the low-potential grid. By contrast, the side beam-guiding holes of the high-potential grid are eccentric to those of the low-potential grid. The beams 6B, 6B and 6R passing through the beam-guiding holes is converged on the center region of the phosphor screen 3. The horizontal magnetic field generated by the yoke 8 is shaped like a pincushion, whereas the vertical magnetic field generated by the yoke 8 is shaped like a barrel. The electron beams 6B, 6G and 6R deflected by the pincushion-shaped and barrel-shaped magnetic fields are converged at any region of the phosphor screen 3.
In the self-convergence in-line-type color cathode-ray tube, an electron beam is influenced by astigmatism after passing an uneven magnetic field. For instance, the beam is distorted as shown in FIG. 2A. The beam spot, which the beam forms on a peripheral region of the phosphor screen, is inevitably distorted as shown in FIG. 2B. The electron beam is also affected by deflection aberration, which occurs when the electron beam is focused excessively in the vertical direction, generating a large halo 13 extending in vertical direction as shown in FIG. 2B. The larger the cathode-ray tube, the greater the deflection aberration. The larger the angle by which the beams are deflected, the lower the image resolution at the peripheral regions of the phosphor screen.
Means for preventing the image resolution from lowering due to deflection aberration is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, Jpn. Pat. Appln. KOKAI Publication No. 61-250934, and further, in Jpn. Pat. Appln. KOKAI Publication No. 2-72546. As shown in FIG. 3, the electron gun assemblies comprise first grid G1 to fifth grid G5, and an electron beam generator GE, a four-pole lens QL, and a final focusing lens EL, which are arranged along the axes of electron beams. As shown in FIGS. 4A and 4B, the multiple lens, for example, four-pole lens QL has three electron beam guide holes 14a, 14b, and 14c in one end of the third grid G3 and three electron beam guide holes 15a, 15b and 15c in that end of the fourth grid G4. The multiple lens, for example, four-pole lens QL and the final focusing lens EL change in synchronism with the magnetic field of the deflecting yoke. This makes it possible to prevent the electron beams from being distorted at the peripheral regions of the screen, despite of the deflection aberration of the deflecting magnetic field. Thus, the beams can form undistorted spots on any region of the screen.
If such a mean is used, however, a problem arises when the astigmatism caused by the deflecting yoke is very strong at the peripheral region of the screen, though the halo extending in a line perpendicular to the beam spot. Namely, it is not possible to eliminate the sideways expansion of the electron beam spot.
This problem with the conventional electron gun assembly will be explained with reference to FIG. 5. FIG. 5 illustrates the lens operation performed in the conventional electron gun assembly. In FIG. 5, the solid lines represent the track of an electron beam, showing how the lens focuses the beam at the center of the phosphor screen. The broken lines represent the track of the electron beam, illustrating how the lens focuses the beam at a peripheral region of the screen.
As shown in FIG. 5, the multiple lens, for example, four-pole lens QL1 is provided on the cathode side of the main electron lens (EL). To direct the electron beam to the center of the screen, only the main electron lens EL indicated by the solid lines focuses the electron beam. To deflect the electron beam to the peripheral region of the screen, a deflecting lens DYL is formed by the deflecting magnetic field represented by the broken lines.
Generally, a self-convergence-type deflecting magnetic field is generated in a color cathode-ray tube. The force for focusing the beam in the horizontal direction H does not change, and the deflecting lens DYL focuses the beam in the vertical direction V only.
FIG. 5 does not show the action of the magnetic field for deflecting the beam in the horizontal direction, for the purpose of illustrating only the problem caused by the self-convergence, deflecting magnetic field.
When the deflecting lens DYL is formed, that is, when the embodiment is focused at a peripheral region of the screen, the force of the electron lens EL is decreased as shown by the broken lines in FIG. 5. To compensate for the force of the lens EL for focusing the beam in the horizontal direction H, the multiple lens QL1 is formed. As a result, the electron beam travels along the track shown by the broken lines and is focused at the peripheral region of the screen. The main plane of the lens for focusing the electron beam in the horizontal direction H is at position A when the electron beam is directed at the center of the screen. (The main plane is the virtual center of the lens, or a point at which the track of the emitted beam crosses that of the beam radiated onto the screen.) When the electron beam is deflected to the peripheral region of the screen, forming a multiple lens, the main plane extending in the horizontal direction H moves to position B and lies between the main electron lens EL and the multiple lens QL1. Further, the main plane extending in the vertical direction V moves from the position A to position C. Therefore, the main plane extending in the horizontal direction H moves back from the position A to the position B, decreasing magnification. Furthermore, the main plane extending in the vertical direction V moves forward from the position A to the position C, increasing the magnification. Consequently, a difference emerges between the magnification in the horizontal direction and the magnification in the vertical direction. The electron beam spot formed in any peripheral region of the screen inevitably expands sideways, or in the horizontal direction.
It is an object of the present invention to provide a color cathode-ray tube in which the sideways expansion of a beam spot is eliminated or reduced, despite of the difference in magnification between the horizontal and vertical lenses, and which can therefore form undistorted beam spots in all regions of the screen.
According to a first aspect of this invention, there is provided a cathode-ray tube comprising:
an electron beam formation portion for forming and emitting electron beam;
an electron gun assembly having a main electron lens section for accelerating and focusing the electron beam; and
a deflecting yoke for generating a deflecting magnetic field for deflect-scanning the electron beam emitted from this electron gun assembly in the horizontal and vertical directions on a screen; wherein
the main electron lens section comprises at least four electrodes provided in the order of first, second, third and fourth grids, a middle first voltage being applied to the first grid, an anode voltage being applied to the fourth grid, the adjacent second grid and the third grid being connected by a resistor, second and third voltages which are higher than the first voltage and lower than the anode voltage, being applied to the second and third grids; a first lens region being formed the first grid and the second grid; a third lens region being formed between the third grid and the fourth grid; a second lens region being formed between the second grid and the third grid; and an asymmetrical lens being provided in this second lens region.
Furthermore, according to this invention, there is provided a cathode ray tube wherein the lens power of the first, second and third lens regions changes in synchronism with the deflecting magnetic field.
Moreover, according to this invention, there is provided a cathode ray tube characterized in that, as the electron beam is directed from the center portion of the screen toward the peripheral region of the screen in synchronism with the deflecting magnetic field, the first and third lens regions have a lens power which weakens in the horizontal and the vertical directions, and by contrast, the asymmetrical lens provided in the second lens region has a lens power of relatively focusing in the horizontal direction and diverging in the vertical direction. That is, when the electron beam is in the center of the screen, the electron gun assembly according to an embodiment of the present invention has a diverging action in the horizontal direction and a focusing action in the vertical direction, and when the electron beam is at the peripheral region of the screen, the electron gun assembly has a focusing action in the horizontal direction and a diverging action in the vertical direction.
Furthermore, according to this invention, there is provide a cathode ray tube is wherein a voltage which changes in synchronism with the deflecting magnetic field is applied to the first grid, and as the electron beam is directed from the center portion of the screen toward the peripheral region of the screen, in synchronism with the deflecting magnetic field, the first and third lens regions have a lens power which weakens in the horizontal and the vertical directions, and by contrast, the asymmetrical lens provided in the second lens region has a lens power of relatively focusing in the horizontal direction and diverging in the vertical direction, thereby canceling overall changes of the lens power in the horizontal direction of the first and third lens regions.
Furthermore, according to this invention there is provided a cathode ray tube wherein, by applying an AC voltage which changes in synchronism with the deflecting magnetic field to the first grid, the AC voltage components thereof are applied via static capacitances between the first grid, the second grid, the third grid and the fourth grid to the second grid and the third grid, thereby changing the lens power of the first, second and third lens regions.
Furthermore, according to this invention there is provided a cathode ray tube wherein a voltage which changes in synchronism with the deflecting magnetic field is applied to the first grid, the second grid is electrically connected to a fifth grid, and the fifth grid is provided adjacent to the first or another grid to which a voltage which changes in synchronism with the deflecting magnetic field is applied.
FIG. 6 shows the electron beam track and lens power of the above constitution. Here, the solid line represents the electron beam track and lens power when the electron beam is focused in the center of the screen, and the broken lines represents the electron beam track and lens power when the electron beam is focused at the peripheral region of the screen. In the electron gun assembly according to the present invention as shown in FIG. 6, the multiple lens, for example, four-pole lens (QL1) is positioned substantially near the center of the main electron lens (EL), and when the electron beam is directed at the center of the screen, this multiple lens (QL1) has a lens power of focusing in the vertical direction and diverging in the horizontal direction, and when the electron beam is deflected toward the peripheral region of the screen, it has a lens power of focusing in the horizontal direction and diverging in the vertical direction, as shown in the diagram by the broken line. Furthermore, when the electron beam is directed at the center of the screen, since the multiple lens (QL1) functions as a diverging lens in the horizontal direction and as a focusing lens in the vertical direction, the main electron lens (EL) is a substantially cylindrical lens of strong focusing strength in the horizontal direction, so as to compensate the horizontal and vertical focus difference. Then, this main electron lens (EL) becomes weaker over its entirety when the electron beam is deflected to the peripheral region of the screen, and in the horizontal direction, it operates so as to cancel the lens operation of the preceding multiple lens (QL1). At this time, the track of the electron beam in the vertical direction is like that shown by the broken line, but the track of the electron beam in the horizontal direction is not different from when the electron beam is focused in the center of the screen, since the position of the multiple lens (QL1) roughly matches the position of the main electron lens. Therefore, the lens main plane (hypothetically the lens center; the cross point between the emitted beam track and the beam track radiated onto the screen) which focuses the electron beam in the horizontal direction (H) does not change whether the electron beam is in the center of the screen or deflected to the peripheral region of the screen (main plane Axe2x80x2=main plane Bxe2x80x2), and in the vertical direction, although the main plane position moves forward by the amount generated by the DY lens, in comparison with the conventional electron gun assembly, with the conventional electron gun assembly, the multiple lens (QL1) is positioned closer to the cathode side than the main electron lens, and the multiple lens (QL1) generates divergence in the vertical direction, and the electron beam track passes a position distant from the core axis of the main electron lens (EL), and the main plane position C was moved forward by that amount, but in the electron gun assembly of the present invention, since the multiple lens (QL) is provided inside the main electron lens (EL), the track of the electron beam entering the main electron lens (EL) is unchanged, and consequently the shift position (main plane Cxe2x80x2) of the main plane in the vertical direction is further forward (on the cathode side) by that amount than the main plane position C of the conventional electron gun assembly, the magnification in the vertical direction being no greater than the conventional electron gun assembly, and the vertical diameter of the electron beam at the peripheral region of the screen does not greatly deteriorate. Therefore, in comparison with the conventional electron gun assembly, in the electron gun assembly of the present invention, the main plane position has little deviation in the horizontal and vertical directions at the peripheral region of the screen, and the phenomenon of sideways deviation of the electron beam at the peripheral region of the screen is reduced by that amount, achieving a more rounded electron beam. Consequently, by using the electron gun assembly according to the present invention, it is possible to obtain a cathode ray tube with no sideways deviation at the peripheral region of the screen and better resolution in all regions of the screen. Moreover, the second grid and the third grid are connected at a resistor provided near the electron gun assembly, and since the second grid and the third grid are provided between the first grid, to which an AC voltage in synchronism with the deflecting magnetic field is applied, and the fourth grid, to which a DC anode voltage is applied, the components of the AC voltage applied to the first grid can be applied to the second grid and the third grid via the static capacitances between the first grid, the second grid, the third grid and the fourth grid, and the multiple lens formed between these electrodes can be operated using the potential difference between the second grid and the third grid generated at this time. Furthermore, the resistor provided near the electron gun assembly applies a voltage, obtained by resistance-dividing the anode voltage Eb applied to the fourth grid, to the second grid and the third grid, and therefore it is not necessary to apply an extra voltage from outside the cathode ray tube, making it easy to realize a high-quality cathode ray tube as shown above.